Throughout contemporary manufacturing, there are myriad occasions when products must be scrupulously cleaned of various deleterious materials prior to further processing or use. In the area of electronics manufacturing, those unwanted materials may be the masking chemicals or soldering flux residue materials common to printed circuit board production and assembly. When the problem of cleaning is aggravated by the irregularities of components assembled to a board and the close spacing of the mounted components, special cleaning devices, solutions and processes are needed.
The traditional means of cleaning such assembled printed circuit boards is by way of various chlorinated hydrocarbon or chlorofluorocarbon based solvents. These solvents are recognized today to be undesirable because of the various health and environmental hazards they create, most particularly the threat to the Earth's protective ozone layer. In contemporary society, more and more stress is being placed on solving problems attendant to environmental protection, and thus, the drive to substitute environmentally neutral manufacturing processes escalates.
Attempts have been made to utilize water as a solution in cleaning printed circuit boards. In the traditional water-cleaning processes, when water is sprayed angularly at the surface of a printed circuit board, the water bounces off. This means that only the spot actually impacted by the water spray is cleaned, and the rest of the surface is merely wetted. When components are added to the board so that it becomes important or necessary to remove tough and tenacious residue from under these components, this "spray-and-bounce" method is especially ineffective. If the water is sprayed vertically downward, it's momentum is dissipated as the stream impacts the surface and the flow direction is abruptly changed. The resultant sideward flow lacks the force necessary to dislodge flux residue in small spaces.
U.S. Pat. No. 3,868,272, issued Feb. 25, 1975 to Tardoskegyi, entitled CLEANING OF PRINTED CIRCUIT BOARDS BY SOLID AND COHERENT JETS OF CLEANING LIQUID discloses a process of cleaning a liquid flux from the surface of a printed circuit board having components mounted thereon using through-hole technology. Tardoskegyi utilizes a plurality of closely spaced nozzles for directing high velocity jets of cleaning liquid perpendicularly against the entire upper and lower surfaces of a printed circuit board.
With the technology utilized in electronic device assembly today, particularly in computer devices, lead length between components has been shortened, resulting in greater density of mounted components on printed circuit board substates, thus creating a tightly crowded, difficult to clean assembly.
One recent manufacturing development that has enabled dense packaging is the surface mounting of components upon printed circuit boards. Surface mount technology was developed in response to the need to connect components with lead spacing closer than was practical with conventional through-hole techniques. This technology also allows for the mounting of discrete, leadless devices which have very small clearance between their lower surfaces and the adjacent surface of the printed circuit board to which they are mounted. The surface mount approach solves the problem of drilling tightly spaced holes into the printed circuit board and allows closer component placement, but requires the use of a solder flux in a paste form rather than liquid form. Frequently, the paste flux will contain the solder in suspension.
During the assembly process, when the printed circuit board with surface mounted components is exposed to heat to melt and flow the solder, the flux paste tends to polymerize into a firm resinous material. The removal of this material is difficult, but essential. Unless complete cleaning is accomplished, electrical failure in the form of short circuits is much more likely due to the fact that the flux includes droplets of solder, which is conductive. Alternatively, the hardened flux residue, if not totally removed, can serve as an insulating layer to block conductive portions of the electrical circuit from contact by a test probe, thereby interfering with necessary testing of the printed circuit board assembly.
Therefore, in accordance with an aspect of the present invention, a feature is to provide a new and improved method and apparatus for cleaning the surfaces of printed circuit boards with sufficient effectiveness to dislodge and remove hardened and trapped particulate matter of the paste flux resulting from a surface mount process.
In accordance with another aspect of the present invention, a feature is to provide a new and improved method and apparatus for cleaning the surfaces of printed circuit boards with the use of water or a water based cleaning solution.